There is a current trend toward the use of a laser beam to provide guidance for certain vehicles, thereby eliminating the need for an attendant to guide them. Such a system is described in co-pending U S. Pat. No. 4,790,402, which is owned by the assignee of the present invention.
U.S. Pat. No. 4,790,402 describes a vehicle on which is mounted a laser unit. This transmits a scanning beam which is intercepted and reflected by one of a series of retroreflective targets mounted along the intended pathway of the vehicle. The reflected beam returns to the laser unit on the vehicle and provides a course-corrective signal by means of which the vehicle is caused to follow a desired path.
U S. Pat. No. 4,647,784 describes another method by which a vehicle is navigated. This vehicle operates in an area in which are installed a plurality of retroreflective targets at known locations. The vehicle carries a laser unit which transmits a scanning beam. This strikes and is reflected back from two or more concurrently visible targets, and these reflected signals provide bearings which permit the vehicle's location to be determined by triangulation that is performed by an on-board computer.
In working with the vehicle of U.S. Pat. No. 4,790,402 it has been found that direct sunlight striking the laser unit can cause a false signal which may result in malfunctioning of the guidance system. Sunlight contains many frequencies, including the one which is being transmitted from and reflected back to the laser, so a beam of sunlight striking the laser may cause the same reaction as the laser beam reflected from a target. Since the target and the sunbeam are not necessarily at the same bearing from the vehicle, an undesired heading for the vehicle may result. It is surmised that a similar problem may be encountered by other vehicles utilizing a laser in their navigation. It also seems possible that strong light sources other than sunlight could cause false signals in a laser unit.